The mechanical action of straining, by which all particles larger than the interstices of the porous material are arrested, is one important function of filters, and it used to be commonly represented as their only function. They may act, however, in other ways to purify water. Not to speak of the further mechanical actions of subsidence on upper surfaces of particles of the filtering medium, and lateral attraction and adhesion of suspended matter (which doubtless occur in some measure), it is an important fact, now well ascertained, that a filter may separate from water a portion of the matter held in solution. On the other hand, it is often precisely such matter, when of organic origin and putrescent (and minute invisible disease-germs may here be included with the matter in solution), that it is specially desirable to remove from drinking water, as being prolific of mischief when taken into the system. In numerous instances an outbreak of virulent disease, such as typhoid fever, has been clearly traced to water so contaminated. The danger is the greater that such water may be bright and sparkling, and peculiarly palatable.

That even sand has the power of removing dissolved matter from water was shown by Mr Witt’s valuable experiments at Chelsea, described in 1856. In one of these, e.g., water containing 1 ’ 42 grains of chloride of sodium per gallon (70,000 grains) was deprived of 22 per cent, of that substance by filtration through a depth of 1 foot 9 inches of sand. The sand had no appreciable action on dissolved organic matter, as charcoal had, but the quantity of such matter originally present was small. It is probable that all finely porous material has such action, more or less. The efficiency of charcoal in this respect, and especially fresh animal charcoal, has been well demonstrated and utilized. The mode of the disappearance of dissolved organic impurities has been a disputed point. Some say they are retained and accumulated in the pores of the charcoal; but the experimental evidence seems to leave little doubt that they are mainly oxidized, so as hardly to impair the activity of the filter. In fact, the value of a filtering material will be found to depend chiefly on the power it has of bringing oxygen, stored in its fine pores or otherwise provided, into chemical union with the dissolved organic matter and destroying it. At the same time it is obvious that, chiefly by the mechanical action of straining, organic matter may accumulate in a filtering medium, and in course of time, through decomposition, render the water which passes through more impure and less wholesome than in its unfiltered state.

We may remark here that river water and shallow well water, while extensively used for water supply, are in general the most largely polluted. Rivers commonly receive large volumes of sewage, impure surface water from cultivated land, and other contamination. The water of shallow wells, especially in large towns, or when near churchyards, stables, cesspools, <fcc., is often contaminated with organic matter of the worst kind, in large quantity. While rain water, collected from the roofs of houses in butts, no doubt contains organic matter, this hardly bears comparison in amount to the organic impurity in rivers (thus Dr Hassall found it a hundredfold less). Deep spring water, again, is freed from much of its organic impurity through natural filtration. The advantages of a good lake-supply for large towns has been amply demonstrated in these days, notably in the case of Glasgow, which draws its water from Loch Katrine.

Though the arrangements for water supply of most of our large towns include filtering processes by means of which, as a rule, excellent drinking water is abundantly provided, so that in the opinion of some chemists a domestic filter may be superfluous, while it is sometimes a source of harm (owing to lack of proper attention), it is generally thought a wise additional safeguard to employ one of those instruments, in view, more especially, of some of the possible consequences of epidemics and floods, and the necessity of housestorage of the water received. In country places, and in various other circumstances, their use is often quite imperative if the laws of health are to be respected.

The Japanese and Egyptians seem to have used water filters at an early period. These consisted of sandstone or unglazed earthenware, and were of bowl or egg shape, with small Drojections at the top resting on a wooden frame. The water poured into this vessel filtered through to a vessel below. About the middle of last century, slabs of a lias found in Picardy were used as a filtering agent, being fitted as a false bottom in water cisterns; the water was drawn off through a tap from the space below. A porous filtering stone of Teneriffe was at one time imported largely into England. The “alcarrhazas” are filter vessels of porous biscuit stoneware made in Spain.

One of the earliest filters in England was that patented by Mrs Johanna Hempel in 1790. It was a supported basin made of tobacco pipe (or similar) clay and coarse sea, river, drift, or pit sand, and hardened in the furnace. In the following year (1791) the ascending principle was first applied by Mr James Peacock. Water from an upper reservoir was admitted through a pipe to the bottom of a box containing strata of sand, gravel, and a mixture of charcoal and powdered calcareous stone. Passing up through this, it was drawn off by a pipe at the top. The filter was occasionally cleaned with an exhausting and condensing pump, which sucked up water rapidly through the filtering material and then sent it back with force, washing out the dirt.

The construction of filters is a matter on which inventiveness has been largely exercised. All sorts of porous substances have been called into requisition, as may be seen by a glance at the patent records. Thus, to mention some of these, we have various kinds of stone, sand, gravel, powdered glass, clay, porous sulphur, preparations of iron, charcoal (vegetable and animal), cloth, felt, horsehair, skins, paper, silicated carbon, sponge, wood, wool, cane, capillary threads, and so on. Vegetable charcoal, we may note, was first employed in 1802, animal charcoal in 1818, and solid carbon blocks in 1834.

It is impossible here, and it would be tedious, to give anything like a detailed account of the changes that have been rung on the filtering principle for domestic filters. In the simplest and most familiar forms, of course, the water passes down by its own gravitation through the filtering medium to a reservoir below. The force of downward pressure has sometimes been augmented by a head of water, sometimes by a force pump, and sometimes by means of air condensed above the water to be passed through the filter. Or the air has been extracted from the vessel containing the filtered liquid, thus adding force to the atmospheric pressure above. The ascending principle has appeared in various applications; and in some cases the water has been caused first to descend and then to ascend through filtering material (a vertical partition, e.g., being fixed in a vessel so as to reach nearly to the bottom, and layers of sand and vitrified limestone being placed on either side of the lower part; the water is poured in on one side of the partition and drawn from the other side near the top). Lateral filtration has also been tried. Filters have been arranged to act in the cistern, or in connexion with the service pipe between the cistern and the water tap, or independently, and, in the latter case, either having the unfiltered water poured into them, or being placed in a vessel of it, and giving filtered water through a tube. Sometimes a series of separate connected vessels have been employed; and for very dirty water it is often advantageous to have one system of filtration for the coarser, and another for the finer impurities. Once more, filters have been rendered self-supply in g by means of a ball-cock. These are some of the general forms which the filter has taken.

The application of pressure to filters cannot as a rule be pursued very far, for it tends to derange the apparatus and render the filtered liquid muddy. Enlargement of surface is a better means of obtaining rapid filtration. Upward filtration, while it offers some advantages over downward, has not hitherto come very much into use. It is open to objection in that the water sent upwards has a tendency to force a passage through certain channels, without being uniformly disseminated in the material; and the deposit of any filth is excluded from view, and mostly also from smell, instead of being exposed and giving us warning.

In passing now to examine more closely some of the approved forms of domestic filter at present in use, it should be borne in mind that while any of these filters will doubtless purify water both mechanically and chemically, more or less, it is only on condition of their being properly attended to, and the filtering material renovated at intervals depending on its nature and the nature and amount of impurity in the water. The term “self-cleansing,” applied to some filters, may have a (limited) true sense, but if understood to imply that a filter, let alone, will go on ad infinitum giving pure water, it is quite inapplicable; solid impurities must accumulate and call for removal. The statement, occasionally made, that a filter is “warranted to remove all impurities” from water is absurd and hardly deserves notice. Absolutely pure water is a thing almost unknown; careful distillation alone will give an approximation to it. Again, the claim that a filter will remove all lime from water is often false; filtration is capable of removing only a small quantity of lime. It must be allowed that sundry points in the process of filtration still remain in some obscurity, and it is matter of regret that the action of some common filtering agents has not been so fully cleared up by scientific experiment as others. Still enough has been ascertained probably to guide to the construction of a filter on rational principles.

In a large proportion of filters, as already indicated, some form or other of carbon is the chief filtering agent. The well-known filters of Lipscombe are cylindrical-shaped covered vessels of glazed earthenware, in which the filtering medium, a mixture of vegetable and animal charcoal, in granular form, is enclosed between two slabs cemented in the case. The upper (glazed earthenware) slab has a central aperture with raised border, and a small perforated basin immediately below it; into this is inserted a sponge to arrest the grosser impurities, which is taken out and cleaned at short intervals. The filtered water passes through the lower (and porous) slab to the reservoir below, which communicates above with the outer air by a narrow tube passing up within to the top of the apparatus, and delivers its water through a tap.

Charcoal in the form of solid finely porous blocks, which can be conveniently brushed and cleaned externally, is now often moulded for filters. The convenient decanter filter, in which the water passes through the block to a central tube, forms an elegant addition to the sideboard. The annexed figure (1) represents one of these as made by Atkins, who also furnishes earthenware filter vessels having a division across the inside wherein a carbon block is fitted water-tight, which can readily be taken out and cleaned and replaced, or a fresh one substituted. Sometimes the block is fitted in a movable pan. Again, in the filter shown in fig. 2 a double filtration is effected, the water passing first through loose charcoal B, then through a charcoal block C, supported as shown. The block in this case is said to last longer without cleaning. The movable and perforated earthenware plate A, which is placed above the charcoal (see fig. 3), allows of easy renewal of the latter. The charcoal used in these filters is chiefly of vegetable origin. They are found to remove more or less of organic and inorganic matter dissolved in water.

It may be useful here to call attention to some of the conclusions arrived at regarding charcoal in the sixth report of the Royal Commission on Rivers’ Pollution a short time ago. Fresh animal charcoal has been proved to act powerfully in the removal of organic impurity (considerably more so than vegetable charcoal), as well as of mineral matter. But, according to Dr Frankland, its reduction of the hardness ceases in about a fortnight, the removal of organic matter continuing even after six months, though to a much less extent, especially if the filter be much used. It was found necessary to renew the charcoal every six months when used for filtering the comparatively pure water of the London New River Company, whilst the water of the Thames required renewal of the charcoal every three months. If this be not done, myriads of minute worms, we are told, are developed in the material passing out with the filtered water. Other statements, of scientific weight, regarding animal charcoal are more favourable to it, and seem to show that under certain conditions, perhaps imperfectly understood, it may give better results.

In Major Crease’s system, which is adopted in the British army and navy, loose animal charcoal is compressed between plates by means of a screw, the amount of compression being determined by the degree of impurity in the water to be filtered.

The silicated carbon of Mr Dahlke’s filters is obtained by mixing animal or vegetable charcoal with the residue of distillation of Boghead coal. By adding a little clay to the latter product, and saturating the whole with oily matter, it can be moulded, after which it is burnt. In one form of the filter, two carbon blocks are sealed into the interior of earthenware vessel, granulated carbon being placed between them; in another, a double action is obtained by placing a carbon block over the entrance to a second carbon medium. These filters have been highly commended for their chemical properties, attributed to magnetic oxide of iron present in the medium. We give (fig. 4) a representation of the silicated carbon siphon filter with its case. Water may be sucked through it from a stream directly into the mouth, or passed siphon-wise from one vessel into another. These and similar pocket filters of Atkins’s were supplied to the Ashantee expedition.

A very powerful filtering medium was discovered and introduced many years ago by Mr Thomas Spencer. It is called magnetic carbide, and consists of protoxide of iron in chemical combination with carbon. It is obtained by roasting hematite iron-ore with granulated charcoal for twelve to sixteen hours at a dull red heat. Mr Spencer considers the purifying property of the oxide to be due to its power of attracting oxygen to its surface, without tho latter being acted upon, the oxygen attracted being then changed into ozone, by which the organic matter of the water is consumed. The magnetic carbide is used in granular form. This filter gained prize medals at the last London and Paris exhibitions, and its efficiency was demonstrated by the Lancet Sanitary Commissioners’ report on filters in 18G7.

The only other system we shall hero notice is that in which spongy iron is used. This substance is metallic iron which has been reduced from an oxide without fusion. It is in a spongy or porous state of extremely fine division. Its remarkable purifying action on water was discovered by Professor Gustav Bischof of the Andersonian University, Glasgow; and experiments made with his filters by the Royal Commissioners already referred to showed that their power both of removing organic matter and reducing the hardness of water even increased during upwards of eight months’ constant use. The general form of the filter is represented in fig. 5. An inner vessel containing tho spongy iron is supported in a case, which, below, contains some prepared sand, a regulator A, and a receptacle C, for filtered water (with tap, not shown). The unfiltered water B is in this form supplied from a bottle which is inverted into the upper part of the inner vessel (a method familiar to chemists). After passing through the spongy iron, the water ascends through an overflow pipe in the direction of the arrows; the object of this is to keep the spongy iron, when once wet, constantly under water, as otherwise it is too rapidly oxidized. The object of the prepared sand (which is generally in three layers, viz., pyrolusite at the top, then sand, then gravel) is to separate traces of iron retained in solution. The regulator A consists of a tin tube, cemented in the position shown; it is open at the inner end, which is below the perforated bottom supporting the sand, and closed by a screw cap at its outer end. It has also a small lateral perforation, through which alone the filtered water passes into the reservoir. Should the perforations get choked, the screw cap is removed, and a brush inserted; on starting at first, too, the · cap is unscrewed, that the materials may be well washed out without soiling the lower reservoir. With a ball-cock and constant supply of water, the inner vessel is dispensed with.

The nature of the action of the metal on organic matter is rather obscure. Mr Bischof considers there are both reducing and oxidizing agencies constantly at work, and that the oxides of iron, being present in their nascent state, must be very energetic in their action. Probably ferric hydrate, the last product of oxidation, takes an active part in separation of the organic matter, transferring oxygen to it. Again, spongy iron is known to be very energetic in precipitating any lead or copper. Its reduction of the hardness of water presents some difficulty.

This filter, we may add, recently gained the prize medal for general excellence given by the Sanitary Institute of Great Britain, and has been otherwise commended.

The subject of cistern filters (of which there are many varieties) need not detain us long here. The arrangement adopted by Lipscombe, shown in fig. 6, may be taken as an example. The impure water passes through the inlet into the chamber of the filter, thence upwards through a plate of porous stone, then through powdered charcoal into the pure water reservoir, whence it may be drawn off 1 cold by the pure water tap, or hot and pure from the boiler.

When the filter is in action, the grosser impurities are stopped by the porous stone, while the finer pass into the charcoal and are chemically acted on. Each time the unfiltered water tap is opened to obtain water, unfiltered water enters the inlet and scours out the impurities in the chamber. To clean the filter, once every six months or other period, a plug is drawn up by means of a galvanized chain (attached as shown) from the top of the filter and dropped into the inlet. The unfiltered water tap is then turned on several minutes, so that the cistern water rushes down through the filter and out of the tap, carrying out impurities from the filter.

In the filtration of water for supply of towns, galleries of masonry are often constructed in the sand and gravel forming the river bed or banks; the water percolates through and enters these tunnels at the bottom and by side channels; it is thence pumped to the town. Genoa, Toulouse, Lyons, and Perth present examples of this system, which is apt to prove rather costly. The system of artificial filter-beds of sand resting on gravel, &c., is now more generally adopted. It was first introduced into London by Mr Simpson, in 1839, after a study of various works in the north, and especially of the costly experience of Glasgow. For the Chelsea Water Company...